linux-kernel - Re: [PATCH 2/3] function-graph: enable the stack after initialization of other variables

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Message-ID: <alpine.DEB.2.00.0906021528210.14994@gandalf.stny.rr.com>
Date:	Tue, 2 Jun 2009 15:30:14 -0400 (EDT)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
cc:	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>, stable@...nel.org,
	"Luis Claudio R. Goncalves" <lclaudio@...g.org>,
	Oleg Nesterov <oleg@...sign.ru>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH 2/3] function-graph: enable the stack after initialization
 of other variables


On Tue, 2 Jun 2009, Frederic Weisbecker wrote:
> > diff --git a/kernel/trace/trace_functions_graph.c b/kernel/trace/trace_functions_graph.c
> > index d28687e..baeb5fe 100644
> > --- a/kernel/trace/trace_functions_graph.c
> > +++ b/kernel/trace/trace_functions_graph.c
> > @@ -65,6 +65,12 @@ ftrace_push_return_trace(unsigned long ret, unsigned long func, int *depth)
> >  	if (!current->ret_stack)
> >  		return -EBUSY;
> >  
> > +	/*
> > +	 * We must make sure the ret_stack is tested before we read
> > +	 * anything else.
> > +	 */
> > +	smp_rmb();
> 
> 
> Isn't this part a too much costly for very traced function?

I was thinking that, but otherwise we still have the problem. It is a read 
barrier which I don't think is as costly as a write or full barrier. But 
because we have the if statement, perhaps a "read_barrier_depends" can do?

[ added Paul McKenney because he's good with barriers ]

-- Steve


> 
> 
> > +
> >  	/* The return trace stack is full */
> >  	if (current->curr_ret_stack == FTRACE_RETFUNC_DEPTH - 1) {
> >  		atomic_inc(&current->trace_overrun);
> > -- 
> > 1.6.3.1
> > 
> > -- 
> 
> 
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